The present invention refers to a focusing system of the light beam for a videoprojector comprising at least an image microforming device DMD (Digital Micromirror Device). Videoprojection systems based on image microforming devices DMD (Digital Micromirror Device) are spreading more an more, above all for their excellent image quality, in particular for their brightness and resolution, as well as for the small size of projectors, compared e.g. to the devices using kinescopes.
Shortly, an image microforming device DMD consists of a set of aluminum square mirrors with a 16 μm side, each one of them forming an element of the image to be projected, i.e. a pixel. The mirrors can rotate by ±10 degrees around a diagonal, and the rotation in either direction is produced by the action of two electrodes located under the mirror on opposite sides with respect to the rotating axis. The light hits the mirror with an angle of about 20 degrees with respect to the perpendicular at the mirror plane, when the latter is in its “rest” condition, i.e. when it is not attracted by one of the two electrodes. When the mirror is rotated in one direction, the reflected ray will be deflected and not enter in the projection lens; consequently it is not sent to the screen, i.e. the pixel is “off”; if the rotation occurs in the opposite direction, then the pixel is “on”, since the reflected light is sent to the screen.
To each pixel is associated a cell of a static memory of the type SRAM (Static Random Access Memory), containing all information for controlling the electrodes that ensure mirror rotation. Even if the light reflected by each mirror always has the same intensity, changing the time during which a pixel remains on will cause brightness to change due to an integrating action produced by the human eye.
A videoprojector may comprise one, two or three image microforming devices DMD. The solution utilizing only one image microforming device DMD is particularly advantageous for its low-cost, overall dimensions and weight; the present invention is mainly related to this solution.
In order to obtain a colored image when using only one DMD device, the mirrors of the image microforming device DMD are illuminated sequentially by the three primary colors, i.e. red, green and blue, which are obtained sending the light of the lighting lamp to a revolving wheel, called color wheel, divided at least in three sectors, each one consisting of a dichroic filter apt to produce one of the three primary colors. By means of the rotation of the color wheel, the light beam sent to the image microforming device DMD will take all three different colors sequentially.
In the videoprojector the light beam requires focusing. This is usually obtained by means of a convergent lens system, also known as “relay lens”; the focusing plane can be obtained on the entry of the projection lens, according to the known Kohler's configuration; alternately, the focusing plane can be localized on the surface of the image microforming device DMD, according to the critical or Abbe's illumination configuration. The latter configuration has the advantage to obtain the vertical centering of the image on the projection screen, since the image size on the image microforming device DMD is big enough to ensure a ±6 mm displacement of the projection lens in a perpendicular direction to the projection axis without loosing its brightness. In the instance of a frontal projection, this permits the vertical displacement of the image on the screen by an amount about equal to the height of the image itself.
The focusing systems know, using at least three converging lenses, have the drawback of a poorly defined image quality, mainly due to chromatic aberration phenomena, spherical aberration, as well as a further aberration phenomenon called curvature of field. Moreover, said convergent lenses generally produce considerable image distortions. As known, the chromatic aberration phenomenon is due to the focusing plane of the light beam crossing a convergent lens depending on the wavelength of the light beam components, so that a blue light beam, for example, converges on a plane nearer to the lens, whereas a red light beam will converge on a farther plane; a green light beam will focus on an intermediate plane between the planes associated to the blue light beam and red light beam. In the instance of a videoprojector having only one image microforming device DMD, focusing on the surface of the image microforming device DMD, according to Abbe's illumination configuration, is the result of a compromise between the focusing surfaces of the three primary colors. The same considerations apply in the instance of Kohler's configuration, where the beam converges at the entry of the projection lens. The spherical aberration phenomenon occurs when a light beam crosses a convergent lens, in which case the so-called marginal rays of said light beam, i.e. the rays crossing the lens in its peripheral area, will converge to a nearer point to the lens plane of the so-called paraxial rays, that cross the lens in the central area. Also in this instance focusing is the result of a compromise between the central or paraxial rays and peripheral rays. The curvature of field phenomenon is due to the fact that the image focused by the lens is correctly focused on a curved surface rather than on a plane. Also in this instance, being the mirrors of the image microforming device DMD arranged on a flat surface, focusing is the result of a compromise. Finally, the term distortions indicates geometric deformations of the image, the most important ones being the so-called “tilt” deformation, which occurs e.g. when a rectangle projected through the lens system takes the shape of a parallelogram, the distortions know as cushion distortion and barrel distortion.
In order to reduce the above aberrations and distortions, appropriate lens systems are usually suggested, where each effect is corrected by a special optical device, with the result to increase the number of lenses, and the consequent higher projecting complexity and costs.